This [SIS imports not overwriting passwords] is by design.
The most common workflow is for the SIS import to set an initial password, but then if people change it through the UI, you want that to have precedence, even if the override stickiness is checked.
Some schools keep uploading the same SIS file over and over, perhaps with changes. If the SIS import overwrote the password, there would be a lot of people who couldn't login anymore. That's not a good user experience.
If you don't want people to be able to change their passwords through Canvas then you could use an external authentication system or remove the Change Password link (see How to remove "Change Password" profile option ). That's not a real fix, though -- it just makes it harder for them to find it.
You can override the password through the API, which is a less automatic, less likely to run as a bulk process over and over with a password still stuck in there.